Spartan In A Spin

Immediately after reaching orbit and setting up Columbia for two weeks of research, Chawla checked out the RMS in readiness for the planned Spartan deployment a

little over a day into the mission. It was at this point that problems emerged. One of its scientific objectives was to conduct simultaneous observations of the solar corona with SOHO, as part of efforts to recalibrate the latter's instruments and ensure that its sensitivity had not degraded after two years in space. However, on the very day that STS-87 was launched, SOHO suffered a voltage surge that temporarily disrupted its pointing control system.

The billion-dollar satellite is in solar orbit, 1.6 million km closer than Earth's own orbital course, and it was hoped that the joint experiment with Spartan could allow both spacecraft to gather data from their different vantage points.

Shortly before going to bed on the evening of 19 November, Kregel's crew was advised of SOHO's problem and informed of the decision to postpone Spartan's deployment until the 21st, by which time it was expected that SOHO would be back up and running. "We plan to execute our [observing] programme as planned,'' said astronomer Richard Fisher, one of Spartan's principal investigators. "It will just start one day later.'' By the afternoon of 20 November, SOHO was back on-line, although Mission Operations representative Lee Briscoe explained that even in the case of further problems, the deployment would have gone ahead regardless.

"There's both a science programme and a joint science programme,'' added Fisher. "I can't exactly quantify for you what percentages [between them] are, but we would go ahead and complete the Spartan-specific science programme and we'd

Kalpana Chawla controls the RMS from her station on Columbia's aft flight deck during Spartan-201 proximity operations.

expect there would be no disruption of that. And that would give us a considerable yield. We would be very happy with that.'' Late the following night, Chawla duly grappled Spartan to begin the deployment procedure and released the satellite at 9:04 pm, as Columbia flew over the Pacific Ocean. What happened next, however, would dog her career.

''We pick up the Spartan and take it to its deploy location,'' she had told an interviewer before launch. ''It's critical that the location be right, because it's supposed to be looking at the Sun at that time. At that time we let it go, and we watch it for about two minutes when it does a pirouette. That tells us that all the automated tasks that are supposed to go on have started on time. If that does not happen, then we'll take it back and put it in the bay for another try on another day.''

After releasing the satellite, the crew was surprised when it failed to perform its pirouette, implying a problem with its attitude-control system which provided fine pointing towards solar targets. Had Chawla, responsible for supervising Spartan's pre-deployment checkout, missed issuing a critical command? It was a question that would be revisited in the coming weeks.

''The command that we didn't receive onboard the spacecraft would have powered down some of the systems that were up in a standby health-check mode,'' said Spartan Mission Manager Craig Tooley. ''We basically power up a bunch of subsystems and check their status and then bring them back down into a quiescent state to await deploy[ment], where the sequence is then picked up again and the flight programme kicks in. That step - that brought those down into a quiescent state and had them waiting for deployment - wasn't received by the spacecraft.

''Therefore, the attitude-control system was still there warming up, but not configured to deal with the error signals from the sensors or fire some pyrotechnic gas valves that would have turned on the thrusters. So it was basically 'idle' and it never got out of 'idle' mode.'' Chawla duly moved in to regrapple the satellite, but did not receive a firm 'capture' indication on her control panel in Columbia's aft flight deck and backed the arm away. In doing so, she accidentally bumped Spartan and imparted a two-degrees-per-second rotational spin on it.

''Houston, we have a tip-off rate. It's rolling around,'' Kregel told Mission Control. ''We will go ahead and match it and regrapple.'' For much of the next hour, he used Columbia's RCS jets to manoeuvre his ship in such a way that its rotation matched that of Spartan, so that Chawla could try again to regrapple it. The concern was one of timing: a second deployment attempt had to occur within an hour of the first to prevent a timing device from automatically closing the fuel valves to the satellite's pointing system.

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